The present invention relates to a brake fluid composition for a vehicle, which is used in a brake device for a vehicle system, the brake fluid composition containing a solvent, a metal corrosion inhibitor, and an antioxidant. More particularly, the present invention relates to a brake fluid composition for a vehicle, capable of improving the metal corrosion-inhibiting capability by containing a glycol mixture as a solvent, a mixture of triazole and thiadiazole as an anti-corrosive agent, an antioxidant, and a stabilizer.
Brake fluid plays an important role of accurately transferring the pressure generated from a master cylinder to a wheel cylinder. Problems occurring during this procedure cause deterioration in brake responsiveness. The brake fluid needs to meet several requirements associated with its chemical and physical properties. Of these, the first requirement is a high equilibrium reflux boiling point (ERBP). The brake fluid itself is difficult to boil. However, the brake fluid has a high temperature at the time of braking, and thus may boil under particular circumstances. If the brake fluid boils, the pressure of the master cylinder may not be accurately transferred, so a stable brake force cannot be expected. Meanwhile, the temperature of frictional heat caused by the frequent use of a disk brake in a brake system is about 800° C. The brake fluid receiving this high-temperature heat is thermally oxidized, resulting in degradation in the metal corrosion-inhibiting capability, causing safety accidents. The second requirement is a high wet equilibrium reflux boiling point. The brake fluid, which is a hygroscopic liquid, is required to have low hygroscopic property, but it is important to prevent the drop in the boiling point of the brake fluid even when the brake fluid absorbs moisture. The reasons are that when the brake fluid absorbs moisture in the atmosphere and thus lowers its boiling point, this may lead to vapor lock, causing safety accidents. In addition, the viscosity change of the brake fluid needs to be small even within a wide temperature range. In addition, a metal corrosion inhibitor and an oxidation stabilizer, which can inhibit the corrosion of various kinds of metals present in the braking device to enhance their durability, are added to the brake fluid.
In the case of the generally used brake fluids, only a glycol ether compound is used as a solvent, or about 30-50 wt % of a boron ester compound is added to the solvent. The brake fluid containing only the glycol ether compound absorbs moisture in the atmosphere if used for a long period of time, and thus lowers its wet boiling point, resulting in the vapor lock, causing a risk of the brake failure which may lead to an accident. Moreover, the metal corrosion-inhibiting capability of this brake fluid is poor. Also, the brake fluid with about 30-50 wt % of a boron ester compound raises its equilibrium reflux boiling point and wet boiling point by using the boron ester compound, and thus has a higher degree of safety than the brake fluid using only the glycol ether compound. However, this brake fluid may corrode metal components by a boronic acid, which is deposited due to hydrolysis of the boron ester compound when moisture is absorbed. The protection of metals and nonferrous metals against the corrosion by these brake fluids can be achieved by an additive for corrosion inhibition and an antioxidant.